1. Technical Field
The present disclosure relates to power circuits, and particularly to an extensible switching power circuit capable of providing corresponding matching currents according to different power loads.
2. Description of Related Art
Switching power circuits are widely used in electronic devices to convert alternating current (AC) provided by power supplies to direct current (DC) used by the electronic devices. Referring to FIG. 9, one such switching power circuit 40 generally includes a rectifier filter circuit 41, a transformer 42, a switch 43, a pulse width modulation (PWM) controller 44, a feedback circuit 45, a half-wave rectifier D1, and a capacitor Cout. The transformer 42 includes a primary winding L1 and a secondary winding L2. AC power supply (AC/IN) is connected to one end of the primary winding L1 through the rectifier filter circuit 41. Another end of the primary winding L1 is grounded through the switch 43 and a protective resistor (not labeled). One end of the secondary winding L2 is connected to one pole of the capacitor Cout through the half-wave rectifier D1, and another end of the secondary winding L2 and another pole of the capacitor Cout are both grounded. The output connector Vo of the switching power circuit 40 is connected between the half-wave rectifier D1 and the capacitor Cout. The PWM controller 44 is connected to the switch 43 to turn the switch 43 on and off. The feedback circuit 45 is connected between the PMU controller 43 and the output connector Vo.
When the switching power circuit 40 is used, AC provided by the AC power supply is converted to DC by the rectifier filter circuit 41, and input to the primary winding L1. Also referring to FIG. 10, the PWM controller 44 generates a controlling electric potential to periodically turn the switch 43 on, where the DC periodically passes through the primary winding L1, such that the DC is converted to a square wave DC. The square wave DC passing through the first winding L1 generates an alternating induction current in the secondary winding L2. The inducing current is filtered by the half-wave rectifier D1 and the capacitor Cout to be converted to DC, and then output from the output connector Vo for use. When the power load of the switching power circuit 40 changes, the PWM controller 44 can detect the change through the feedback circuit 45, and regulate the length of time the switch 43 is turned on and off according to the current load. Thus, the value of the induction current generated in the secondary winding L2 and the DC output from the output connector Vo can be correspondingly regulated. In this way, the switching power circuit 40 can provide corresponding DC according to different power loads.
In the switching power circuit 40, the capacitance of the capacitor Cout is generally configured to be greater than 100 pF for fully filtering out possible AC mixed in the output DC. However, when the induction current generated in the secondary winding L2 is filtered by the capacitor Cout, the charging/recharging process of the capacitor Cout may generate an obvious ripple current. Referring to FIG. 10, despite the electric potential provided by the rectifier filter circuit 41 (V1) and the electric potential on the primary winding L1 (VL1) being even square wave voltages, the electric potential of the output connector Vo (Vout) undulates due to the ripple current. When the length of time the switch 43 is turned on and off is regulated, the ripple current is correspondingly changed. By regulating the output DC according to different power loads, the ripple current adversely influences the stability of the output current and electric potential, and may damage the switching power circuit 40.
Referring to FIG. 11, another conventional switching power circuit 50 includes a rectifier filter circuit 51, a PWM controller 52, four transformers TA, TB, TC, TD, four diodes TD1, TD2, TD3, TD4, four capacitors C1, C2, C3, C4, four switches Q1, Q2, Q3, Q4, and a feedback circuit 53. The transformers TA, TB, TC, TD respectively include primary windings L11, L21, L31, L41 and secondary windings L12, L22, L32, L42. Each of the primary windings L11, L21, L31, L41 has an end connected to the rectifier filter circuit 51. The other ends of the primary windings L11, L12, L13, L14 are respectively grounded through the switches Q1, Q2, Q3, Q4 and protective resistors (not labeled). Each of the secondary windings L12, L22, L32, L42 has an end grounded. The other ends of the secondary windings L12, L22, L32, L42 are respectively connected to the anodes of the diodes TD1, TD2, TD3, TD4. The cathodes of the diodes D1, D2, D3, D4 are all connected to the output connector Vo of the switching power circuit 50, and are also respectively grounded through the capacitors C1, C2, C3, C4. The PWM controller 52 is connected to the switches Q1, Q2, Q3, Q4 to control them periodically on/off. The feedback circuit 53 is connected between the PMU controller 52 and the output connector Vo.
When the switching power circuit 50 is used, AC provided by AC power supply is converted to DC by the rectifier filter circuit 51, and is input to the primary windings L11, L21, L31, L41. The PWM controller 52 generates different controlling electric potentials to respectively turn the switches Q1, Q2, Q3, Q4 on periodically, and the transformers TA, TB, TC, TD function similar to the aforementioned transformer 42. In the switching power circuit 50, the controlling electric potentials can be regulated to maintain the stability of the output current and electric potential. Also referring to FIG. 12, for example, the PWM controller 52 generates two square wave controlling electric potentials (VG1, VG2) to respectively periodically turn on the switches Q1, Q2. The phases of VG1, VG2 are partially staggered. Correspondingly, two square wave DC having staggered phases pass through the primary windings L11, L21, and induction electric potentials (VL12, VL22) having staggered phases are generated in the secondary windings L12, L22. In use, the DC passing through the primary windings L11, L21 may generate corresponding back electromotive forces (back EMF) in the secondary windings L12, L22. The directions of the back EMF are opposite to that of the induction electric potentials (VL12, VL22). When the back EMF and the induction electric potentials (VL12, VL22) are alternately generated in the secondary windings L12, L22, the back EMF may generate corresponding energy gaps (Vinv1, Vinv2) in the induction electric potentials (VL12, VL22). The energy gaps (Vinv1, Vinv2) may generate undulations in the output current and electric potential of the switching power circuit 50. However, the phases of the two induction electric potentials (VL12, VL22) are staggered, and thus VL12 and VL22 can compensate each other's energy gaps (Vinv1, Vinv2) when superimposed on the output connector Vo. Thus, the undulation of the electric potential on the output connector Vo (Vout) can be decreased, such that the switching power circuit 50 can provide an even DC. The feedback circuit 53 can be used similar to the feedback circuit 45 to detect the power load.
The switching power circuit 50 can provide an even DC, and the capacitances of the capacitors C1, C2, C3, C4 do not need to be greater than 100 pF. However, conventional PWM controllers, such as the PWM controller 52, can only turn at most four switches, such as the switches Q1, Q2, Q3, Q4, periodically on and off. Thus, the switching power circuit 50 can provide at most four output currents superimposed on the output connector Vo. The four currents may have difficulty to precisely compensating each other's gaps, and thus the undulations of the output electric potential of the switching power circuit 50 are difficult to fully remove.
Therefore, there is room for improvement within the art.